The present invention relates to double-headed piston type compressors used in vehicle air conditioners. More particularly, the present invention pertains to a structure for suppressing discharge pulsations of refrigerant gas.
A typical double-headed piston type compressor has a drive shaft supported in a housing. The housing includes a pair of front and rear cylinder blocks secured to each other and front and rear housings. The front housing is coupled to the front end of the front cylinder block with a valve plate arranged in between. In the same manner, the rear housing is coupled to the rear end of the rear cylinder block with a valve plate arranged in between. A crank chamber is defined between the cylinder blocks. Further, suction and discharge chambers are defined in each of the front and rear housings. The cylinder blocks also include a plurality of cylinder bores. Each bore in the front cylinder block is aligned with one of the bores in the rear cylinder block. A double-headed piston is reciprocally housed in each pair of cylinder bores. Compression chambers are defined in each cylinder bore between the end of the piston and corresponding valve plates. The number of cylinder bores is represented by n, and the number of compression chambers in each of the front and rear cylinder blocks is also represented by n. A swash plate is fixed to the drive shaft and rotates integrally with the shaft. Rotation of the swash plate is converted into linear reciprocation of each piston. The reciprocation of each piston compresses the refrigerant gas in each compression chamber.
During operation of the compressor, compressed refrigerant gas is constantly discharged from the compression chambers to the discharge chambers. The pressure in each discharge chamber is momentarily increased every time refrigerant gas is discharged thereto from one of the compression chambers. This periodically pulsates the pressure in the discharge chambers thereby generating so-called discharge pulsation. When analyzing the discharge pulsation using a fast Fourier transform (FFT), it is apparent that the pulsation includes a wide variety of frequency components ranging from zero cycles to a large number of cycles. Among the frequency components, the main component is the n-cycle component, which corresponds to the number n of the cylinder bores. The n-cycle component corresponds to the vibration component that occurs n times during one rotation of the drive shaft. When the compressor is operated at a normal speed, the n-cycle frequency component tends to be close to the natural frequencies of various auxiliary devices, such as an alternator, which are coupled to the compressor by a belt. In this case, resonance occurs in the auxiliary devices and thus noise in the passenger compartment is increased.
Japanese Unexamined Utility Model Publication No. 60-84779 discloses a compressor having a structure for suppressing discharge pulsation. In this compressor, the proximal end of a pipe is connected to each of the front and rear discharge chambers. The pipes have substantially the same length. Openings at the distal ends of the pipes face each other in the cylinder block.
In this compressor, discharge pulsation is suppressed by equalizing the lengths of the two pipes, which function as discharge passages, and by causing the refrigerant gas streams discharged from the pipes to collide with each other. However, the apparatus of this publication does not suppress the n-cycle frequency component, which is the main cause of vibration and noise.
If the rate of decrease in the amplitude of the pulsation of refrigerant gas discharged from the front compression chambers is different from that of refrigerant gas discharged from the rear compression chambers as shown in FIG. 7(b), the n-cycle frequency component is not sufficiently suppressed. In the compressor of the above publication, refrigerant gas discharged from the two pipes (discharge passages) is merged in a chamber such as a discharge muffler before flowing out to an external refrigerant circuit. The phase of the n-cycle frequency components in the refrigerant gas discharged from one of the pipes is different from the phase of the n-cycle frequency components in the refrigerant gas discharged from the other pipe. The difference of the decrease rates of pulsations causes the amplitudes of n-cycle frequency components to be different in the refrigerant gas discharged from the two pipes. In this case, two n-cycle frequency components having different phases and different amplitudes exist in the pulsation of refrigerant gas in the single chamber. Therefore, the compressor of the above publication cannot adequately suppress the n-cycle frequency component.